In recent years, projection television is spreading as a means for producing a television image on a large screen. In attaining the quality of the image, the performance of the projection lens plays an important role.
The projection lens for a projection television needs to have a substantial nominal focal ratio (with an F-number close to 1) to secure the "brightness" of the lens, and, in addition, must be capable of good correction of aberration to the periphery of the screen. A problem associated with the projection television is its depth, which is larger than that of the direct-viewing television (ordinary television set with the CRT fluorescent surface seen directly), so a projection lens with a shorter projection length is desired.
To achieve a high image-forming performance, a large nominal focal ratio and a large field angle with a conventional projection lens formed solely of glass lens elements, the number of lens elements, and the aperture must be increased, and the cost is therefore increased. Moreover, the weight of the entire lens system is increased. Furthermore, correction of the performance in the periphery of the screen is difficult. If aspheric plastic lens elements are employed, correction of the aberration to the periphery is facilitated. However, if the aspherical amount is increased to increase the corrective power, the precision in the fabrication is lowered, and the design performance is not fully realized. In particular, when the aspheric lens is provided near the screen, because of the large light beam diameter, the precision error easily cause aberration. Thus, due to the occurrence of the flare, the contrast is lowered. In addition, the refractive index and the shape of the plastic lens vary to a greater extent. Thus, due to occurrence of the defocus, image-forming performance is degraded.